1. Field of the Invention
The present invention relates to acousto-optical tunable filters cascaded together. More specifically, the present invention relates to acousto-optical tunable filters cascaded together and controlled by RF signals generating beats with different phases.
2. Description of the Related Art
Optical communication systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. However, as users require larger amounts of information to be rapidly transmitted, and as more users are connected to the systems, a further increase in the transmission capacity of optical communication systems is required.
Therefore, there is a continual effort in increase transmission capacity of optical communication systems. For example, through improvements in the modulation rate, optical communication systems with modulation rates in the giga-order bits per second (Gb/s) rate are now in practical use. However, optical communication systems having a transmission capacity of tera-order bits per second (Tb/s) may be required for an optical communication system to handle future demands, such as those imposed by the transmission of images. Improvements in the modulation rate will not, by itself, be enough to handle these future demands.
Therefore, wavelength division multiplexing (WDM) is becoming an indispensable technique for increasing the transmission capacity of optical communication systems. With WDM, a plurality of wavelengths (or “channels”), each carrying information, are multiplexed together and transmitted through a single optical fiber as a WDM signal. This transmitted WDM signal is then received and demultiplexed back into individual wavelengths, so that the information can be obtained from the individual wavelengths. In this manner, a plurality of wavelengths are transmitted through a single optical fiber. This can be contrasted to conventional approaches where a single wavelength is transmitted through a single optical fiber.
Some WDM optical communication systems require wavelength multiplexing such that a few wavelengths to about one-hundred (100) wavelengths are multiplexed together over a wide band. Also, some WDM optical communication systems require wavelength intervals as wide as 1 nm to tens of nm.
Acousto-optical tunable filters (AOTF) are a type of optical wavelength filter that is becoming indispensable in WDM optical communications systems. With an AOTF, wavelength characteristics of the filter can be controlled by changing an RF signal applied to the AOTF, to thereby provide a selectively tunable wavelength filter. AOTFs will be very useful in optical components such as optical add/drop multiplexers (ADM), optical cross-connects, and optical switches.
For example, FIG. 1 is a diagram illustrating an optical ADM node. Referring now to FIG. 1, an optical ADM node 100 receives a light 102 which includes optical wavelength-multiplexed signals of wavelength 1 to wavelength 8. The wavelength-multiplexed light 102 is input to a separator 104 in optical ADM node 100.
Separator 104 separates wavelength 1 to wavelength 4 from wavelength-multiplexed light 102 and permits wavelength 5 to wavelength 8 to pass therethrough.
The lights passing through separator 104 are input to a coupler 106. Coupler 106 couples lights having wavelengths 1′ to 4′ with the lights of 5 to 8, and outputs a wavelength-multiplexed light 108 via an output node.
An acousto-optical tunable filter (AOTF) can be used as separator 104 or coupler 108 to arbitrarily change the wavelengths to be separated or coupled, and to arbitrarily change the number of wavelengths to be separated or coupled. As a result, it is easy to modify the system configuration by external control.
FIG. 2 is a diagram illustrating a polarization-independent type AOTF, which is one type of AOTF. With this type of AOTF, the main axis of the refractive index of a waveguide is rotated in response to light of the wavelength corresponding to the frequency of a surface acoustic wave (SAW). Hence, a rotation of the polarization of the propagating light makes it possible to extract or modulate a particular wavelength.
Referring now to FIG. 2, optical waveguides 11 and 12 are formed on a LiNbO3 X-cut substrate by diffusing Ti therein. A transducer 15 that generates a SAW corresponding to an RF signal (radio frequency signal that is an electromagnetic wave equal to or lower than 3000 GHz) is formed on optical waveguides 11 and 12.
In order to extract wavelengths 1 to 4, RF signals with four frequencies corresponding to the coupled wavelengths 1 to 4 are applied to transducer 15.
An input light 1 having the wavelengths 1 to 8 is applied to a waveguide type polarization beam splitter (waveguide type PBS) 16, which separates input light 1 into a TE-mode light and a TM-mode light. The TM-mode light enters optical waveguide 11, and the TE-mode light enters optical waveguide 12.
The polarization of the light of the wavelengths (wavelength 1 to wavelength 4 in the case shown in FIG. 1) corresponding to the SAWs is rotated from the TM-mode light to the TE-mode light in optical waveguide 11, and is rotated from the TE-mode lights to the TM-mode light in optical waveguide 12.
A waveguide type PBS 17 outputs the TM-mode light in optical waveguide 11 to a pass-through light side and outputs the TE-mode light to a branching light side. Further, waveguide type PBS 17 outputs the TE-mode light in optical waveguide 12 to the pass-through light side and outputs the TM-mode light to the branching light side. Hence, a particular wavelength (wavelength 1 to wavelength 4) can be extracted or modulated.
Absorbers 19 and 20 are SAW absorbers that prevent the SAWs from being reflected by an end surface of the substrate.
The AOTF shown in FIG. 2 can also be used as the coupler shown in FIG. 1. In this case, for example, wavelength 1′ to wavelength 4′ are input as input light 2, while wavelength 5 to wavelength 8 from the separator are input as input light 1.
Then, waveguide type PBS 16 causes the TM-mode light of the wavelength 5 to wavelength 8 to be incident to optical waveguide 11 and causes the TE-mode light thereof to be incident to optical waveguide 12.
When the RF signals corresponding to wavelength 1′ to wavelength 4′ are input to transducer 15, the polarization of the corresponding lights is changed from the TE-mode light to the TM-mode light in optical waveguide 11, and is changed from the TM-mode light to the TE-mode light in optical waveguide 12.
Waveguide type PBS 17 outputs the TM-mode light in optical waveguide 11 to the pass-through light side and outputs the TE-mode light to the branching light side. Further, waveguide type PBS 17 outputs the TE-mode light in optical waveguide 12 to the pass-through light side and outputs the TM-mode light to the branching light side.
Unfortunately, with the conventional use of an AOTF, the output can undesireably vary with time. For example, in a case where the AOTF is used to extract a plurality of wavelengths (such as wavelength 1 to wavelength 4), if a plurality of frequencies are applied to transducer 15 of the AOTF, the central frequencies of the band-pass/band-rejection characteristics deviate from the target frequencies with time. Hence, the output of the AOTF varies with time although the input light has a constant power.